Why is it हमारे घर and not हमारा घर? Does the honorific plural extend to the whole sentence?
Because of में. हमारे is oblique case, which is used when there is a post position. में is a post position.
This may help better explain: http://www.learning-hindi.com/post/1133921729/lesson-50-postpositions
I would say that पे instead of में would make this mean, 'at' our house instead of 'in' our house.
Edit: I would put it in the same place, before हैं, and not modify the rest of the sentence.
Basically, yes. Except it is पे, which is a colloquial form of पर (at). Granny is at our house. If you really want to specify "in," you should say में.